Effect Of High Thermal Stability Aluminides On Microstructure And Mechanical Properties Of Heat-resistant Al-Si Alloys

The high-temperature mechanical properties of traditional aluminum alloys can not meet the heat-resistant requirements of the new engine with service temperature above 300?.It is an important way to improve the high temperature mechanical properties of heat-resistant aluminum alloy by introducing strengthening phase with high strength,high thermal stability,high volume fraction and good combination with aluminum matrix.In this paper,eutectic Si,Al₃Ni,Al₃Zr and Al₃Ti reinforced particles were produced by eutectic reaction and melt in-situ reaction by adding K₂Zr F₆ salt and K₂Ti F₆ salt into the Al-Si-Ni matrix alloy melt.The influence law and mechanism of Si content,salt addition and heat treatment process on the microstructure,room temperature and high temperature mechanical properties of Al₃Zr&Al₃Ti/Al-Si-Ni,Al₃Zr/Al-Si-Ni and Al₃Ti/Al-Si-Ni composites were investigated.The main results are as follows:(1)The microstructure of Al₃Zr/Al-8Si-2Ni composites or Al₃Ti/Al-8Si-2Ni composites are mainly composed of?-Al,eutectic Si,Al₃Ni,and(Al,Si)₃Zr or(Al,Si)₃Ti.With the addition amount of K₂Zr F₆ or K₂Ti F₆ increase,the amount of(Al,Si)₃Zr or(Al,Si)₃Ti phases increases,and they change from rod like to block,and eutectic Si and Al₃Ni phases are refined significantly.The tensile strength and yield strength of the composites at room temperature and 350?increase first and then decrease with the increase of K₂Zr F₆ and K₂Ti F₆.Among the composites with different Al₃Zr content,the tensile strength and yield strength of 4%Al₃Zr/Al-8Si-2Ni composite at room temperature are the highest,reaching 186.3 MPa and91.8 MPa,respectively.Among the composites with different Al₃Ti contents,the tensile strength of 3%Al₃Ti/Al-8Si-2Ni composite is the highest,reaching 184.5MPa.The tensile strength and yield strength of 4%Al₃Zr/Al-8Si-2Ni composites with different Al₃Zr contents are the highest,reaching 67.8 MPa and 58.8 MPa respectively,which are 31.14%and 24.58%higher than that of the matrix respectively.Among the composites with different Al₃Ti contents,the tensile strength and yield strength of 4.5%Al₃Ti/Al-8Si-2Ni composite are the highest,reaching 70.3 MPa and 58.2 MPa,respectively,which are 35.98%and 23.31%higher than the matrix,respectively.(2)The effects of Si mass fraction,total mole of Al₃Zr&Al₃Ti,Al₃Zr/Al₃Ti molar ratio on microstructure and mechanical properties of Al₃Zr&Al₃Ti/Al-Si-Ni composites were investigated by orthogonal test.The results show that the microstructure of the composites is mainly composed of?-Al,eutectic Si,Al₃Ni and(Al,Si)₃(Zr,Ti)phases.With the addition amount of K₂Ti F₆ and K₂Zr F₆increase,the shape of(Al,Si)₃(Zr,Ti)phase changes from rod to block.With the increase of Si content and Al₃Zr&Al₃Ti total mole,the tensile strength at room temperature increases first and then decreases.The influence of three factors on the tensile strength at room temperature is as follows:Al₃Zr&Al₃Ti total mole>Al₃Zr/Al₃Ti molar ratio.At 350?,the order of influence of each factor on tensile strength is Si content>Al₃Zr&Al₃Ti total mole>Al₃Zr/Al₃Ti molar ratio.(3)The effect of different heat treatment processes on microstructure and mechanical properties of 2%Al₃Zr&3%Al₃Ti/Al-10Si-2Ni composite were investigated.The results showed that the eutectic silicon nodulizing phenomenon occurred when the solid solution temperature of T6 heat treatment is 490?.With solution temperature increasing,the nodulizing aggravating.The precipitated phase Al₃Ni distributed along with the eutectic silicon after aging.At room temperature,with the increase of solution temperature,tensile strength and elongation of the composites kept increasing.After heat treatment 535?×3 h+water quenching(60?)+185?×6 h,tensile strength,yield strength and elongation of the composite reached the highest,which were 11.85%,3.79%and71.54%higher than that of the as-cast.Because the rigid structure was destroyed,the high temperature tensile strength and yield strength always decreased with the increase of solution temperature.After heat treatment 535?×3 h+water quenching(60?)+185?×6 h,the tensile strength and yield strength decreased by 16.64%and 7.93%compared with the as-cast state,respectively,while the elongation increased 56.13%compared with the as-cast state.(4)After heat exposure at 450?,the eutectic silicon and Al₃Ni phases in the microstructure of the Al₃Zr/Al-8Si-2Ni composites were nodulized,and the strengthening effect of the rigid interconnection structure was weakened.The elongation of the composites increased with the increase of heat exposure time;the tensile strength and yield strength decreased rapidly and then tend to be stable.When the heat exposure time was 150 h,the room temperature tensile strength and yield strength of 4%Al₃Zr/Al-8Si-2Ni composite decreased by 31.03%and28.43%,respectively,compared with the as-cast condition.The high temperature tensile strength and yield strength of 4%Al₃Zr/Al-8Si-2Ni composite decreased by 39.62%and 45.58%,respectively,compared with the as-cast condition,and the elongation increased 222.48%compared with the as-cast condition.